Conventionally, a two-light source type optical pickup device is used to perform recording, reproducing or the like (recording, reproducing or both) of information on a DVD (Digital Versatile Disk) and a CD (Compact Disk). The two-light source type optical device includes a laser emitting device for DVD that emits light of the wavelength of approximately 650 nm and a laser emitting device for CD that emits light of the wavelength of approximately 780 nm. Furthermore, in order to reduce the sizes of the respective light sources, a two-wavelength integrated laser emitting device capable of emitting two kinds of wavelengths with one package has become practical. As the two-wavelength integrated laser emitting device, there are known a monolithic laser emitting device having two laser diodes formed on a monolithic semiconductor, a hybrid-type laser emitting device having two bonded semiconductor substrates on which laser diodes are respectively formed, or the like.
In the case of the two-wavelength integrated laser emitting device, respective emitting positions of the two laser diodes (laser diodes for DVD and CD) are slightly apart from each other, and the distance therebetween is approximately 110 μm in general. Therefore, when the optical axis of one of the laser diodes is aligned with a system optical axis passing through an objective lens or a collimator lens of an optical pickup device, the optical axis of the laser light emitted by the other laser diode is shifted from the system optical axis. In this state, it is not possible for a common light detector to receive both return lights emitted by the laser diodes for DVD and CD and reflected by the optical recording medium. Therefore, it is proposed to diffract both of or either of the return lights of the emitted lights from the laser diodes for DVD and CD using a diffraction grating or the like so as to introduce both return lights to the common light detector (see Patent Document Nos. 1 and 2).
Further, as the optical recording medium is recently required to have a large capacity, an optical recording medium such as an optical disk for a blue-violet laser or the like having a capacity several times that of DVD or CD has become practical. As a result, in terms of the reduction in size and cost, it is demanded to enable the recording, reproducing or the like of information on optical recording media (such as DVD, CD, an optical disk for blue-violet laser or the like) using a common optical pickup device. Therefore, a three-light source type optical pickup device having the laser diode for blue-violet in addition to the laser diodes for DVD and CD has been developed.
The following first and second configurations have been proposed as examples of the three-light source type optical pickup device. The first configuration includes three laser emitting devices emitting lights of different wavelengths. Optical axes of the lights (of the different wavelengths) emitted by the respective laser emitting devices are aligned with a system optical axis using prisms provided corresponding to the respective wavelengths, and the lights of the respective wavelengths are introduced to an optical recording medium. The return lights of the three kinds of wavelengths reflected by the optical recording medium are introduced to a common light detector via the respective prisms, and are detected by the light detector (see, for example, a Non-Patent Document No. 1).
The second configuration uses a three-wavelength integrated laser emitting device including three semiconductor substrates emitting lights of different wavelengths λ1, λ2 and λ3 integrated in one package. The emitting position of the light of the wavelength λ1 (405 nm) and the emitting position of the light of the wavelength λ2 (660 nm) are approximately the same, as seen in the direction of the optical axis of the emitted light of the laser emitting device. The emitting position of the light of the wavelength λ3 (785 nm) is apart from the respective emitting positions of the lights of the wavelengths λ1 and λ2 by approximately 110 μm. Among the return lights emitted by the three-wavelength integrated laser emitting device and reflected by the optical recording medium, the return lights of the wavelengths λ2 and λ3 are detected by a common light detector. The return light of the wavelength λ1 is separated by the prism and is detected by another light detector (see, for example, a Non-Patent Document No. 2).
Patent Document No. 1: Japanese Laid-Open Patent Publication No. 2001-143312.
Patent Document No. 2: Japanese Laid-Open Patent Publication No. 2001-256670.
Non-Patent Document No. 1: “Philips, Netherlands, has developed an optical head capable of performing recording and reproducing on CD, DVD and Blu-ray Disk” [online], Jul. 16, 2004, Nikkei BP corporation [accessed on Feb. 20, 2005], internet <http://techon.nikkeibp.co.jp/members/NEWS/20040716/104521/>.
Non-Patent Document No. 2: “An optical head for three-wavelength recording and reproducing corresponding to Blu-ray disk, DVD and CD has been developed” [online], May 17, 2004, Sony corporation [accessed on Feb. 20, 2005], internet <http://www.sony.co.jp/SonyInfo/News/Press/200405/04-026/>.